1. Field of the Invention
The invention relates to polynucleotides encoding a modified HaHB4 transcription factor and polynucleotides encoding functionally active fragments and variants of a modified HaHB4 transcription factor as well as vectors and host cells containing these polynucleotides and the polypeptides encoded by these polynucleotides. The invention also encompasses transgenic host cells (including plant cells), plants, seed, pollen, and plant parts containing the polypeptides and/or polynucleotides of the invention. The invention further encompasses methods of producing transgenic host cells, plants, seed, pollen, and plant parts and the processed plant products produced from these transgenic hosts.
2. Background Art
The homeodomain is generally known to be a conserved DNA binding motif of 61 amino acids present in a subset eukaryotic transcription factors that are involved in regulating developmental processes in higher organisms (Gehring, Science 236: 1245-1252 (1987)). Genes encoding homeodomain containing proteins have been isolated from many eukaryotic organisms including fungi, mammals and plants (Gehring et al., Annu. Rev. Biochem. 63:487-526 (1994)). However, genes encoding proteins that contain a homeodomain associated with a protein interacting leucine zipper domain (often referred to as HD-Zip proteins) have to date, only been found in plants. Homeodomain-leucine zipper proteins are generally believed to be involved in regulating developmental processes associated with the response of plants to environmental conditions (Chan et al., Biochim. Biophys. Acts 1442(1):1-9 (1998), Carabelli et al., Plant J. 4:469-479 (1993); Schena et al., Genes Dev. 7:367-379 (1993)).
HaHB4 is a sunflower (Helianthus annuus) transcription factor that belongs to the subfamily 1 of HD-Zip proteins and shares about 50% amino acid sequence identity within the homeodomain with other members of this sub-family, with the exception of the Arabidopsis thaliana transcription factors AtHB7 and AtHB12, that share 60% and 53% identity, respectively with the corresponding HaHB4 homeodomain sequence HaHB4 is endogenously expressed at very low levels in sunflower plants grown under controlled and normal environmental conditions. The upregulation of endogenous HaHB4 expression in limited water availability and upon exposure to abscisic acid (ABA) is believed to lead to an increased tolerance to the sunflower plant to water stress. Similarly, transgenic Arabidopsis thaliana expressing recombinant HaHB4 have been reported to demonstrate increased tolerance for water stress (drought) conditions. In particular, U.S. Pat. No. 7,674,955 discloses that transgenic Arabidopsis thaliana plants overexpressing HaHB4 display increased tolerance to drought as compared to the wild type variety of the Arabidopsis thaliana plant under the same conditions. However, U.S. Pat. No. 7,674,955 does not disclose the modified HaHB4 transcription factors described herein, or the increased yield of transgenic plants containing these modified HaHB4 transcription factors under water-stressed and non-water stressed conditions as compared to the wild type variety of the plants under the same conditions.
Increasing agricultural yields and the ability of crops to tolerate a wider range of environmental conditions present two approaches for addressing the major challenges faced by world agriculture in feeding an ever growing population on continuously shrinking arable land resources. Accordingly, there is a need to provide new varieties of crops and other plants that display increased crop yield and tolerance to environmental stress.